Delineating neurons that underlie complex behaviors is of fundamental interest. We will explore a novel method for extremely rapid changes in excitability of genetically targeted neurons to affect a robust and vital ongoing regulatory behavior in rodents, i.e., breathing. Breathing is a remarkable behavior that mediates gas exchange to support metabolism and regulate pH. A reliable and robust rhythm is essential for breathing in mammals. Failure to maintain a normal breathing rhythm in humans suffering from sleep apnea, apnea of prematurity, congenital central hypoventilation syndrome, hyperventilation syndrome, Rett syndrome, and perhaps sudden infant death syndrome, leads to serious adverse health consequences, even death. Various neurodegenerative diseases, such as Parkinson's disease, multiple systems atrophy and amyotrophic lateral sclerosis, are associated with sleep disordered breathing that we hypothesize results from the loss of neurons in brain areas controlling respiration. If breathing is to be understood in normal and in pathological conditions, the mechanisms for respiratory rhythmogenesis must be revealed. We focus on a brain site essential for generation of the normal breathing pattern, the preB"tzinger Complex. Using a viral delivery system, we will express genetically encoded rhodopsin-like molecules in various phenotypes of neurons in the preB"tzinger Complex. Rapid changes in excitability of these neurons by administration of light pulses delivered via an optical fiber implanted in the preB"tC in anesthetized, awake or sleeping rats should produce noticeable, even profound perturbations in breathing. Analysis of such perturbations will provide an extraordinary window into understanding mechanisms of respiratory rhythm and pattern generation. PUBLIC HEALTH RELEVANCE: In humans, continuous breathing from birth is essential to life and requires that the nervous system generate a reliable and robust rhythm that drives inspiratory and expiratory muscles. The proposed studies will significantly advance our understanding of the neural mechanisms generating respiratory rhythm and shed light on human disorders of breathing.